For decades researchers, academics and psychologists have wanted to know what kind of person becomes a terrorist. If there are pre-existing traits which make someone more likely to kill for their beliefs – well, that would be worth knowing. In this edition of The Inquiry – part of the BBC World Service Identity Season – we tell the story of that search for a ‘terrorist type’. It’s a story which begins decades ago. But, with the threat from killers acting for so-called Islamic State, finding an answer has never felt more pressing.

(Photo: Somali soldiers stand at the scene of car bomb at a restaurant in Mogadishu, 2016. Militant Islamist group al-Shabab claimed responsibility for the attack. Credit: Getty Images)